Secondhand smoke causes decreased lung function in women, especially those with asthma, according to a study presented at the American Thoracic Society International Conference in Atlanta on Tuesday.

The study of more than 10,000 adults, 440 of whom had asthma, found secondhand smoke did not affect the lung function of men.

"Among females with and without asthma, we found that the greater their exposure to environmental tobacco smoke, the lower their lung function," said Mark Eisner, M.D., M.P.H., Assistant Professor of Medicine at the University of California, San Francisco. "The effect was greatest in women with asthma -- their chronic airway inflammation makes them even more susceptible to the effects of secondhand smoke."

He noted that men were more likely to be exposed to secondhand smoke at work, while women were more likely to breathe in secondhand smoke at home.

"It's possible that men who are more sensitive to environmental tobacco smoke can more easily avoid it at work, while women can't avoid secondhand smoke at home," he said. "Or there may be a biological difference between men and women, such as women's smaller airways, which causes them to be more affected by smoke."

Dr. Eisner said that healthcare professionals who care for people with asthma should assess if patients are exposed to smoke and counsel them to avoid exposure.

The researchers evaluated the relationship between subjects' lung function and cotinine, a substance that is a marker of exposure to secondhand smoke. Most of the subjects showed evidence of exposure to secondhand smoke.

In non-smoking males, there was no evidence that secondhand smoke exposure was related to decreased lung function. But in non-smoking females with or without asthma, high levels of cotinine were associated with lower lung function.

"These findings should provide further evidence for the need for public policies that promote smoke-free environments," Dr. Eisner said.